Impedance matching is often needed in the design of an electronic circuit. Please refer to FIG. 1. Using a super-heterodyne receiver as an example, signals will be received by the antenna 100 firstly. These received signals will go through a bandpass filter 101 so as to filter out the signals outside the desired band secondly. Signals fall within the desired band will go into low-noise amplifier 102 to be amplified thirdly. Signals being amplified will go through an images rejection filter 103. The amplified radio frequency electrical signal will mix with a local oscillating signal 105 in the mixer 104 and be down converted to a lower intermediate frequency signal (IF signal). Finally, the intermediate frequency signal will be processed by an IF signal processing circuit 106. In order to match with the output impedance of the bandpass filter 101, the input impedance of the low-noise amplifier 102 is usually desired to be 50 ohm.
There are mainly 4 different methods proposed in the prior arts regarding the generation of a 50 ohm input impedance of the amplifier. Please refer to FIG. 2(a). In general, the input impedance of a field effect/bipolar transistor 200 is quite high, therefore, the simplest way of generating a 50 ohm input impedance at the input terminal of an amplifier is to connect in parallel a resistor 201 of 50 ohm at the input terminal of the field effect/bipolar transistor 200 to fulfill the requirement of the input impedance matching. Since the resistor 201 is directly and electrically connected to the input terminal of the amplifier, the noises producing by the resistor 201 will result in the direct interferences to the input signals. According to the calculation, the noise figures of the amplifier could be as high as 3 dB, which won't meet the practical requirements, even though the noises produced by the transistor 201 are not counted yet. Besides, the resistor 201 electrically connected in parallel with the input terminal of the amplifier will consume about half of the input power, therefore, this configuration is seldom employed. In FIG. 2(b), it shows the circuit using resistive feedback technique to change the input impedance of an amplifier into 50 ohm. The circuit includes a transistor 202 and resistors 203 to 206. This method is mainly focused on the adjustments of resistors 204 and 205 to produce the input impedance at the input terminal of the amplifier. The noise figure of the amplifier manufactured by this method is lower than the previous method, but the DC power consumption of this configuration is quite large. Referring to FIG. 2(c), it shows the circuit with a common-gate/common-base configuration. The proposed circuit includes a transistor 207 and a resistor 208 to achieve an input impedance of the amplifier of 50 ohm. The main idea of this method is to adjust the transconductance of the transistor 207, gm, and to let gm=20 mS, which will make the input impedance equals to 50 ohm. But through real test, the noises of such a circuit are quite high, and the transconductance of the transistor is also restricted. The last and the most popular circuit for producing an input impedance of 50 ohm of an amplifier is shown in FIG. 2(d). The circuit includes a transistor 209 and an inductor 210 having a terminal connected in series with the source/emitter of the transistor 209. An ideal inductor won't produce any extra noise. Therefore, the lowest noise figure of this configuration will be the same as the transistor 209. In fact, most of the amplifiers with low-noise signals use this kind of configurations. But the inductor 210 will occupy a lot of space on the chip and the quality factor of the inductor on the chip is not high enough, thus a method which could produce a 50 ohm input impedance of the amplifier without using an inductor is really in need.
Keep the drawbacks of the prior arts in mind, and employ experiments and research full-heartily and persistently, the designing methods and circuits for impedance matching are finally conceived by the applicant.